1. Field of the Invention
This invention relates generally to a satellite communications system employing a phased antenna array that provides reduced co-channel interference and, more particularly, to a satellite communications system that employs a phased antenna array having a plurality of antenna elements, where the spatial distribution of the elements has a density taper to reduce beam side lobes and co-channel interference.
2. Discussion of the Related Art
Various communications systems, such as certain cellular telephone systems, cable television systems, internet systems, military communications systems, etc., make use of satellites orbiting the Earth to transfer signals, usually in the form of digital data modulated onto a carrier wave. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then is retransmitted by the satellite to another satellite or to the Earth as a satellite downlink communications signal to cover a desirable reception area depending on the particular use. The satellite is equipped with an antenna system, such as a phased antenna array system, including one or more arrays of antenna elements or feed horns that receive the uplink signals and transmit the downlink signals to the Earth.
FIG. 1 is a schematic block diagram of a transmit phased antenna array system 10 that includes an antenna array 12 having a plurality of array elements 14 for use on a satellite. Each array element 14 includes a phase shifter 16, a high power amplifier 18, such as a traveling wave tube amplifier (TWTA) or a solid-state power amplifier (SSPA), a resistor 20, and an antenna element 22, such as a feed horn. Only seven antenna elements are shown in this example, but as will be appreciated by those skilled in the art, a typical antenna array will include many antenna elements configured in a predetermined geometric pattern, such as a hexagon or circle. The system 10 includes a source 24 that generates a signal to be transmitted. The signal is sent to a beam forming network (BFN) 26 that distributes the signal to each of the separate array elements 14. The phase shifters 16 set each of the separated signals to a predetermined phase progression and the amplifiers 18 amplify the signals for transmission. The antenna elements 22 may also generate beams for other downlink signals.
Each feed horn directs a separate beam at a certain frequency and at a certain beam intensity. A predetermined combination of the feed horns directs a specific downlink signal to a predetermined coverage cell within a reception area. Each downlink signal will include a main lobe directed to the coverage cell and side lobes that may be directed towards the coverage cell of the main lobe of another downlink signal. If the frequency of the two downlink signals is the same, the side lobes may cause co-channel interference (CCI) with the other cell in the reception area depending on the intensity of the side lobes. The CCI needs to be controlled to minimize bit error rate and maximize the channel data rate and system capacity. By reducing the CCI, the isolation between adjacent cells can be increased.
To illustrate this situation, FIG. 2 shows a diagrammatic view of a satellite 30 emitting a plurality of downlink beams 32 from a satellite antenna system 34, such as a transmit phased array (TPA) of the type discussed above, to a reception area 36 on the Earth. The downlink beams 32 include a main lobe 38 and side lobes 40. The main lobes 38 are directed towards a particular cell 42 in the reception area 36. The side lobes 40 may be directed towards the cell 42 for another main lobe. The shape of the combination of the antenna elements 22 transmitting the downlink signal determines the shape of the cell 42. In this view, the cells 42 are circular shaped but other cell shapes can also be generated, as would be understood to those skilled in the art.
The downlink beams 32 are required to be within a particular frequency band based on FFA requirements. Within that frequency band, sub-frequency bands are used to transmit the various beams 32 carrying the digital data. It is desirable to make the sub-frequency bands as wide as possible so that they are able to carry more information, such as for multi-media applications. However, the side lobes 40 of one beam 32 may interfere with the beam 32 for another cell 42 if the beams are using the same sub-frequency band. By using different sub-frequency bands for cells that are adjacent or proximate each other, the CCI can be significantly reduced or eliminated. However, as the bandwidth of the various sub-frequency bands decreases, the amount of information that can be transmitted is limited. Therefore, it is desirable to suppress the side lobes 40 and provide more frequency reuse for adjacent or proximate cells.
For phased array antenna elements, the traditional or conventional technique for reducing beam side lobes and CCI is to employ an amplitude-tapering scheme. In amplitude tapering, the various antenna elements in each array have an output intensity or amplitude that is selected based on its location in the array. Particularly, the centrally positioned antenna elements have the highest intensity output, and as the elements get farther from the center of the array, their intensity output is decreased. Therefore, the elements at the outside of the array have less radiating energy, which reduces the energy of the side lobes, which in turn reduces the co-channel interference for those downlink signals using the same frequency band. Various amplitude tapering algorithms are known in the art for determining the actual intensity output of a particular feed horn depending on its location in the array for different applications. Additionally, by providing a tapered amplitude of the beam in this manner, the width of the main lobe increases.
Amplitude tapering of the type described above suffers from a number of drawbacks. In one amplitude tapering scheme, different power amplifiers are used for the antenna elements to generate the beams of different intensities to establish the amplitude tapering. Because different amplifiers are required for different amplitudes, a wide variety of amplifier designs are employed in each antenna array. However, the cost of the array increases as the number of amplifier designs increases.
In an alternate amplitude tapering scheme, resistors, for example the resistors 20, are used to attenuate the power output of the particular antenna element to provide the amplitude tapering. In this design, each antenna element employs the same amplifier so that the design is consistent, thus realizing cost savings. However, because power on a satellite is an important resource, it is undesirable to throw away power by using attenuating resistors. If the resistor is positioned before the amplifier, the efficiency of the amplifier may be reduced because it does not operate at its saturation point as is desirable.
Although amplitude tapering has been effective for reducing CCI, the drawbacks discussed above have caused phased antenna array designers to investigate additional ways to reduce CCI. It is desirable that all of the power amplifiers be the same for cost efficiency reasons and it is desirable to operate all of the amplifiers in their saturation regions without throwing power away. It is therefore an object of the present invention to provide an improved antenna array to reduce CCI.
In accordance with the teachings of the present invention, a phased antenna array for use on a satellite is disclosed that employs a density tapering technique for positioning the antenna elements in the array to reduce co-channel interference between cells. Particularly, the spatial position of the various antenna elements in the array are spread out so that the center portion of the array has the highest density of elements, and the outer portion of the array has the lowest density of elements. Predetermined schemes are used to set the spatial density of the elements in the array. By providing fewer antenna elements at the outer portion of the array, the beam side lobes are reduced.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.